WO2018232979A1

WO2018232979A1 - Lithium iron phosphate battery

Info

Publication number: WO2018232979A1
Application number: PCT/CN2017/098784
Authority: WO
Inventors: 田少杰; 韩昌隆; 鞠峰; 张翠
Original assignee: 宁德时代新能源科技股份有限公司
Priority date: 2017-06-23
Filing date: 2017-08-24
Publication date: 2018-12-27
Also published as: EP3447838A4; US20200020980A1; CN109119686B; EP3447838A1; EP3447838B1; CN109119686A

Abstract

Disclosed is a lithium iron phosphate battery, comprising: a positive electrode plate including a positive electrode current collector and a positive electrode film provided on the surface of the positive electrode current collector; a negative electrode plate including a negative electrode current collector and a negative electrode film provided on the surface of the negative electrode current collector; a separator provided between the positive electrode plate and the negative electrode plate; and an electrolyte solution containing an organic solvent, a lithium salt and electrolyte solution additives. The electrolyte solution additives comprise a cyclic carbonate containing a double bond and a cyclic disulfonate shown in formula I. In the formula (I), A and B are each independently selected from alkylenes having 1 to 3 carbon atoms. Thus, the present invention can solve the problem of poor wettability between an electrode plate with a high compaction density and an electrolyte solution, improves both the low-temperature performance and the normal-temperature and high-temperature cycle performances of the lithium iron phosphate battery, and effectively extends the service life of the lithium iron phosphate battery.

Description

Lithium iron phosphate battery

Technical field

The present invention relates to the field of batteries, and more particularly to a lithium iron phosphate battery.

Background technique

Lithium ion secondary batteries are widely used in electric vehicles and consumer electronic products due to their high energy density, high output power, long cycle life and low environmental pollution. Lithium iron phosphate is one of the most commonly used cathode materials for power batteries due to its high cycle life, good safety and low price. A disadvantage of lithium iron phosphate batteries is their low energy density. In order to increase the energy density, on the one hand, the gram capacity of the positive and negative electrode materials is increased, and on the other hand, the compaction density of the positive and negative electrode sheets is increased. However, when the compaction density is increased, the diffusion of lithium ions is difficult, and the wettability of the electrode sheet and the electrolyte is deteriorated, so that the cycle life of the lithium iron phosphate battery is reduced. Therefore, it is necessary to improve the performance of the lithium iron phosphate battery under the high-pressure solid-density electrode sheet system from the viewpoint of the electrolyte.

Summary of the invention

In view of the problems in the background art, an object of the present invention is to provide a lithium iron phosphate battery which can solve the problem of poor wettability of a high-pressure solid-density electrode sheet and an electrolyte, and low-temperature performance, normal temperature and high temperature of a lithium iron phosphate battery. The cycle performance is improved, effectively extending the service life of the lithium iron phosphate battery.

In order to achieve the above object, the present invention provides a lithium iron phosphate battery comprising: a positive electrode tab, a positive electrode current collector and a positive electrode film disposed on a surface of the positive electrode current collector; a negative electrode plate including a negative electrode current collector and disposed on An anode membrane on the surface of the anode current collector; a separator disposed between the cathode tab and the anode tab; and an electrolyte containing an organic solvent, a lithium salt, and an electrolyte additive. The positive electrode active material in the positive electrode film includes lithium iron phosphate; and the negative electrode active material in the negative electrode film includes graphite. The electrolyte additive includes a cyclic carbonate containing a double bond and a cyclic disulfonate of Formula I; in Formula I, A and B are each independently selected from an alkylene group having 1 to 3 carbon atoms. base.

Compared with the prior art, the beneficial effects of the present invention are:

The invention can solve the problem that the high-pressure solid density electrode sheet and the electrolyte have poor wettability, and the low-temperature performance, the normal temperature and the high-temperature cycle performance of the lithium iron phosphate battery are improved, and the service life of the lithium iron phosphate battery is effectively prolonged.

Detailed ways

The lithium iron phosphate battery according to the present invention will be described in detail below.

A lithium iron phosphate battery according to the present invention includes: a positive electrode tab, a positive electrode current collector and a positive electrode film disposed on a surface of the positive electrode current collector; a negative electrode electrode plate including a negative electrode current collector and a negative electrode film disposed on a surface of the negative electrode current collector; a separator disposed between the positive electrode tab and the negative electrode tab; and an electrolyte comprising an organic solvent, a lithium salt, and an electrolyte additive. The positive electrode active material in the positive electrode film includes lithium iron phosphate; and the negative electrode active material in the negative electrode film includes graphite. The electrolyte additive includes a cyclic carbonate containing a double bond and a cyclic disulfonate of Formula I. In Formula I, A and B are each independently selected from an alkylene group having 1 to 3 carbon atoms.

In the lithium iron phosphate battery according to the present invention, the cyclic carbonate containing a double bond can improve the capacity retention rate in the high-temperature environment of the lithium iron phosphate battery, but the inevitable problem is that the film formation resistance is increased. It affects the use of lithium iron phosphate battery in low temperature environment; the cyclic disulfonate of formula I can reduce the film formation resistance. Adding the two in combination to the electrolyte can improve the low temperature performance, normal temperature and high temperature cycle performance of the lithium iron phosphate battery, and effectively extend the service life of the lithium iron phosphate battery.

In the lithium iron phosphate battery according to the present invention, the cyclic carbonate containing a double bond may be selected from one or both of vinylene carbonate (VC) and ethylene carbonate (VEC).

In the lithium iron phosphate battery according to the present invention, in the electrolytic solution, the cyclic carbonate containing a double bond may have a mass percentage of 0.5% to 4%. The content is small, the film formation is unstable, and the high-temperature cycle performance of the lithium iron phosphate battery is deteriorated; the content is too large, resulting in excessive film formation, and the lithium iron phosphate battery is prone to lithium precipitation during the circulation process, resulting in cyclic diving. Preferably, the cyclic carbonate containing a double bond has a mass percentage of 0.5% to 3%.

In the lithium iron phosphate battery according to the present invention, the cyclic disulfonate may be selected from methylene methane disulfonate (MMDS), ethylene ethane disulfonate, and subpropylene cethane disulfonate. One or more of the esters.

In the lithium iron phosphate battery according to the present invention, the cyclic disulfonate may have a mass percentage of 0.2% to 2% in the electrolytic solution. The content is small, the reduction of the film formation resistance is not much improved; the content is large, it is easy to crystallize and deposit in the electrolyte, and at the same time, due to its poor high temperature stability, the addition amount may worsen the performance of the lithium iron phosphate battery. Preferably, the cyclic disulfonate has a mass percentage of 0.2% to 1%.

In the lithium iron phosphate battery according to the present invention, the charge cutoff voltage of the lithium iron phosphate battery may not exceed 3.8 V, and preferably, the charge cutoff voltage of the lithium iron phosphate battery may not exceed 3.6 V. This is because vinylene carbonate (VC), ethylene carbonate (VEC) and cyclic disulfonate are unstable at higher voltages, are easily oxidatively decomposed, and have a higher probability of side reactions in the electrolyte. It is large, and in the case where the amount of addition is small, the side reaction product deteriorates the performance of the SEI film formed on the surface of the negative electrode, and therefore, the charge cutoff voltage is not easily excessive. Under such conditions, the electrolyte solution according to the present application has a good film forming effect, a small impedance, and a good wettability of the electrode sheet and the electrolyte, so that the lithium iron phosphate battery containing the high-pressure solid density electrode sheet can be more obviously improved. Low temperature performance, normal temperature and high temperature cycle performance.

In the lithium iron phosphate battery according to the present invention, the negative electrode membrane may have a compaction density of from 1.4 g/cm ³ to 1.8 g/cm ³ . The compaction density is too small, the contact resistance between the powder particles increases, and the overall energy density of the lithium iron phosphate battery is too low; the compaction density is too large, the electrode sheet is easily crushed, and the cycle performance of the lithium iron phosphate battery is deteriorated.

In the lithium iron phosphate battery according to the present invention, the positive electrode membrane may have a compact density of 2 g/cm ³ to 2.5 g/cm ³ . The compaction density is too small, the contact resistance between the powder particles increases, and the overall energy density of the lithium iron phosphate battery is too low; the compaction density is too large, the electrode sheet is easily crushed, and the cycle performance of the lithium iron phosphate battery is deteriorated.

On the surface of the high-pressure solid-state negative electrode sheet, the electrolyte according to the present application can form a denser and more stable SEI film than the conventional electrolyte, and the SEI film has a small impedance, and the electrode sheet and the electrolyte have better wettability. In the lithium iron phosphate battery containing the high-pressure solid-density negative electrode sheet, the electrolytic solution according to the present application can make the lithium iron phosphate battery have better low-temperature performance, normal temperature and high-temperature cycle performance, and the like.

In the lithium iron phosphate battery according to the present invention, the electrolyte may have a conductivity of 25 ° C of 8 mS/cm to 11 mS/cm. If the conductivity is small, the electrolyte kinetic performance is poor, the lithium iron phosphate battery has large polarization, affecting the normal temperature cycle performance and low temperature performance; if the conductivity is too large, the electrolyte thermal stability is poor, resulting in high temperature cycle performance of the lithium iron phosphate battery difference.

In the lithium iron phosphate battery according to the present invention, the electrolyte may have a viscosity at 25 ° C of 2 mPa·s to 4 mPa·s. If the viscosity is too large, on the one hand, the kinetic performance of the electrolyte is poor, on the other hand, the ability of the electrolyte to infiltrate the electrode sheet is lowered, and the performance of the lithium iron phosphate battery is deteriorated comprehensively; if the viscosity is small, the thermal stability of the electrolyte is poor, resulting in a lithium iron phosphate battery. High temperature cycle performance is poor.

In the lithium iron phosphate battery according to the present invention, the kind of the lithium salt is not limited and can be selected according to actual needs. Preferably, the lithium salt may be selected from one or more of LiPF ₆ , LiBF ₄ , LiBOB, LiAsF ₆ , LiCF ₃ SO ₃ , LiFSI, LiTFSI.

In the lithium iron phosphate battery according to the present invention, the organic solvent may be selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, pentene carbonate, 1,2-butanediol carbonate, 2, 3-butanediol carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, acetic acid One or more of ester, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate.

In the lithium iron phosphate battery according to the present invention, preferably, the organic solvent includes a mixed solvent of a cyclic carbonate and a chain carbonate. Such a mixed solvent can be advantageous for preparing an electrolyte having a comprehensive performance such as conductivity and viscosity.

In the lithium iron phosphate battery according to the present invention, it is further preferred that the organic solvent contains a chain carbonate having a methyl group. Still more preferably, the mass percentage of the chain-like carbonate having a methyl group may be 40% or more based on the total mass of the organic solvent of the electrolytic solution. The chain carbonate having a methyl group is preferably one or both of dimethyl carbonate and ethyl methyl carbonate. The chain carbonate having a methyl group is advantageous for further improving the overcharge resistance of the electrolyte. When the mass percentage of the chain carbonate having a methyl group is 40% or more, the comprehensive properties such as electrical conductivity and viscosity of the electrolyte are good.

In the lithium iron phosphate battery according to the present invention, preferably, the organic solvent further includes a carboxylate, and the content of the carboxylate is less than or equal to 30% of the total mass of the organic solvent of the electrolyte. The addition of the carboxylic acid ester can further improve the electrical conductivity and viscosity of the electrolyte, improve the wettability of the high-pressure solid-state electrode sheet and the electrolyte, and further improve the electrochemical performance such as low-temperature performance of the lithium iron phosphate battery. However, if the content of the carboxylate is too large, it will affect the high temperature stability of the electrolyte, deteriorate the high temperature cycle performance of the lithium iron phosphate battery, and at the same time, since the oxidation potential of the carboxylate is lower than that of the conventional cyclic carbonate or chain carbonate. Excessive addition may increase the gas production of lithium iron phosphate batteries.

In order to make the present invention, the technical solutions and the beneficial technical effects of the present invention more clear, the present invention will be further described in detail below with reference to the embodiments. It is to be understood that the embodiments described in the specification are merely illustrative of the invention and are not intended to limit the invention. The formula, proportions, etc. of the embodiments may be selected in accordance with the present invention without substantially affecting the results.

Examples 1-15 and Comparative Examples 1-13 were each prepared in the following manner.

1. Preparation of positive electrode tab

The positive electrode active material lithium iron phosphate, the binder PVDF, and the conductive agent acetylene black are mixed at a mass ratio of 98:1:1, N-methylpyrrolidone is added, and the mixture is stirred until stable and uniform under the action of a vacuum mixer to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated on an aluminum foil, and the aluminum foil was air-dried at room temperature, transferred to a blast oven at 120 ° C for 1 hour, and then subjected to cold pressing and slitting to obtain a positive electrode sheet.

2. Preparation of negative electrode pieces

The negative active material graphite, the conductive agent acetylene black, the thickener carboxymethyl cellulose sodium (CMC) solution, the binder styrene-butadiene rubber emulsion are mixed according to the mass ratio of 97:1:1:1, and deionized water is added. The mixture was stirred until stable and uniform by a vacuum mixer to obtain a negative electrode slurry. The negative electrode slurry was uniformly coated on a copper foil, and the copper foil was dried at room temperature, transferred to a blast oven at 120 ° C for 1 hour, and then subjected to cold pressing. The negative electrode piece is obtained by slitting.

3. Preparation of electrolyte

The organic solvent is a mixed organic solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and methyl propionate (MP). The lithium salt is LiPF ₆ , the content of LiPF ₆ is 12.5% of the total mass of the electrolyte, and finally the cyclic carbonate containing the double bond and the cyclic disulfonate are added. The mass content of each substance in the electrolyte is shown in Table 1. Shown. The conductivity and viscosity of the electrolyte are adjusted by adjusting the ratio of addition of each substance.

4. Preparation of lithium iron phosphate battery

The positive electrode tab, the negative electrode tab and the separator are wound to obtain a cell, and after the cell is placed in the package, the electrolyte is injected and sealed, and the iron phosphate is obtained by a process of standing, compacting, forming, and exhausting. lithium battery.

Table 1 Preparation parameters of Examples 1-15 and Comparative Examples 1-13

Next, the test of the lithium iron phosphate battery will be described.

(1) Low temperature discharge capacity test

At 25 ° C, the lithium iron phosphate battery is first discharged to 2.0V at 1C; then charged to 3.6V at a constant current of 1C, then charged to a current of 0.05C at a constant voltage, and the charge capacity is CC; then the furnace temperature is adjusted to At -10 ° C, discharge with a constant current of 1 C to 2.0 V, and record the discharge capacity as CDT. The ratio of the discharge capacity to the charge capacity is the discharge capacity retention ratio.

The discharge capacity retention ratio (%) of the lithium iron phosphate battery at -10 ° C = CDT / CC × 100%.

(2) Normal temperature cycle test

At 25 ° C, the lithium iron phosphate battery was first discharged at 1 C to 2.0 V and then subjected to a cycle test. The battery was charged at a constant current of 1 C to 3.6 V, then charged at a constant voltage until the current was 0.05 C, and then discharged to 2.0 V with a constant current of 1 C, and thus charged/discharged, and the capacity retention rate of the lithium iron phosphate battery cycled at 25 ° C for 1000 times was calculated.

The capacity retention ratio (%) after the lithium iron phosphate battery was circulated 1000 times at 25 ° C = the discharge capacity at the 1000th cycle / the discharge capacity at the first cycle × 100%.

(3) High temperature cycle test

At 25 ° C, the lithium iron phosphate battery was first discharged at 1 C to 2.0 V and then subjected to a cycle test. The oven is heated to 60 ° C, charged to 3.6 V with a constant current of 1 C, then charged to a current of 0.05 C at a constant voltage, and then discharged to 2.0 V with a constant current of 1 C, thus charged/discharged, and the lithium iron phosphate battery is cycled at 60 ° C for 500 ° C. The capacity retention rate of the second.

The capacity retention ratio (%) after the lithium iron phosphate battery was circulated 500 times at 60 ° C = the discharge capacity at the 500th cycle / the discharge capacity at the first cycle × 100%.

Table 2 Performance test results of Examples 1-15 and Comparative Examples 1-13

It can be seen from Comparative Examples 1-3 that the compaction density of the positive and negative membranes is improved, and the performance of the lithium iron phosphate battery is rapidly decreased without adding MMDS. However, in Examples 1-3, the compaction density of the positive and negative membranes was increased, and the addition of MMDS to the electrolyte significantly delayed the performance degradation of the lithium iron phosphate battery and prolonged the cycle life of the lithium iron phosphate battery. This indicates that in the lithium iron phosphate battery system containing the high-pressure solid-state electrode sheet, the low-temperature performance of the lithium iron phosphate battery can also be adjusted by adjusting the ratio and amount of the cyclic carbonate containing the double bond and the cyclic disulfonate. , normal temperature and high temperature cycle performance are improved.

In Comparative Example 7, the high-pressure solid-state positive electrode membrane and the negative electrode membrane were used, and the infiltration of the electrode sheet and the electrolyte became difficult, resulting in a low capacity retention rate after the normal temperature and high-temperature circulation of the lithium iron phosphate battery. Its cycle life at normal temperature and high temperature. In Comparative Example 3 and Comparative Example 6, VC and VEC were separately added, which can significantly improve the normal temperature and high temperature cycle performance of the lithium iron phosphate battery, but the inevitable problem is that the film formation resistance is increased, which affects the low temperature of the lithium iron phosphate battery. Environment use. It can also be seen from Comparative Example 3-5 that as the VC content increases, the high-temperature cycle performance of the lithium iron phosphate battery is significantly improved, but the low temperature performance and the normal temperature cycle performance deteriorate due to the increased impedance of the formed interface film (SEI film). . In Examples 4 and 8-11, 0.5% of a cyclic disulfonate was further added to the electrolyte, which was effectively reduced by the synergistic action of the cyclic disulfonate and the cyclic carbonate containing a double bond. The film formation resistance significantly improves the low temperature performance and the normal temperature cycle performance of the lithium iron phosphate battery containing the high pressure solid density electrode sheet. In Comparative Example 8, the amount of VC added was too low, the film formation was unstable, and the improvement in the high-temperature cycle performance of the lithium iron phosphate battery was not remarkable. In Comparative Example 9, the VC content was excessive, and the addition of MMDS could not suppress the increase in film formation resistance, and the performance of the lithium iron phosphate battery deteriorated.

In Examples 4-6, as the amount of MMDS added increased, the room temperature cycle performance and low temperature performance of the lithium iron phosphate battery were significantly improved, and the high temperature cycle performance improvement effect was poor. In Comparative Example 10, the amount of MMDS added was too large, which easily precipitated in the electrolyte and affected the quality of the electrolyte. At the same time, it was not consumed in the early stage of the cycle, and may be decomposed into by-products due to its instability, and deteriorated the iron phosphate. The performance of lithium batteries, especially the deterioration of high temperature cycle performance is obvious.

It can be seen from Examples 12-15 that the viscosity and conductivity of the electrolyte can be improved by adjusting the composition of the organic solvent, and the low temperature performance, normal temperature and high temperature cycle performance of the lithium iron phosphate battery can be improved. For example, in Examples 12-14, the content of the cyclic carbonate is controlled to be 30%, and the total amount of the chain carbonate is 70%, since only the chain-like carbonate with methyl group EMC and DMC are used in Example 14. The DEC is not added, so the electrical conductivity and viscosity of the electrolyte are comprehensive, and the overall performance of the lithium iron phosphate battery is also good. In Example 15, a carboxylic acid ester is further added as an organic solvent, which can further improve the electrical conductivity and viscosity of the electrolytic solution, improve the wettability of the high-pressure solid-state electrode sheet and the electrolytic solution, but inevitably affect the lithium iron phosphate. High temperature cycling performance of the battery. In Comparative Examples 11 and 12, the conductivity of the electrolyte was too low and the viscosity was too large, which made it difficult to infiltrate the electrode sheet and the electrolyte, which deteriorated the power performance of the lithium iron phosphate battery, resulting in deterioration of the low-temperature discharge capacity and the normal temperature cycle performance. . In Comparative Example 13, the content of the chain-like carbonate having a methyl group was as low as 20%, and the content of the carboxylate was too high, reaching 50%, resulting in an excessive conductivity of the electrolyte and a too small viscosity. The stability of the electrolyte is deteriorated, which may seriously deteriorate the high temperature cycle performance of the lithium iron phosphate battery. Therefore, the conductivity and viscosity of the electrolyte need to be controlled within a certain range in order to improve the low temperature performance, normal temperature and high temperature cycle performance of the lithium iron phosphate battery.

In summary, the present invention can be used to improve the properties of a lithium iron phosphate battery containing a high pressure solid density electrode sheet. By adjusting the amount of cyclic carbonate and cyclic disulfonate containing double bond and adjusting the composition of the organic solvent system, an electrolyte with superior comprehensive performance can be obtained, and the low temperature performance of the lithium iron phosphate battery can be obtained. , normal temperature and high temperature cycle performance are improved.

The above embodiments may be modified and modified as appropriate by those skilled in the art in light of the above disclosure. Therefore, the invention is not limited to the specific embodiments disclosed and described herein, and the modifications and variations of the invention are intended to fall within the scope of the appended claims. In addition, although specific terms are used in the specification, these terms are merely for convenience of description and do not limit the invention.

Claims

A lithium iron phosphate battery comprising:

a positive electrode tab comprising a positive current collector and a positive electrode membrane disposed on a surface of the positive current collector;

a negative electrode tab comprising a negative current collector and an anode membrane disposed on a surface of the anode current collector;

a separator disposed between the positive electrode tab and the negative electrode tab;

An electrolyte comprising an organic solvent, a lithium salt, and an electrolyte additive;

It is characterized in that

The positive electrode active material in the positive electrode film includes lithium iron phosphate;

The anode active material in the anode membrane includes graphite;

The electrolyte additive includes a cyclic carbonate containing a double bond and a cyclic disulfonate of Formula I;

In Formula I, A and B are each independently selected from an alkylene group having 1 to 3 carbon atoms.
The lithium iron phosphate battery according to claim 1, wherein the charge cutoff voltage of the lithium iron phosphate battery does not exceed 3.8 V, and preferably, the charge cutoff voltage of the lithium iron phosphate battery does not exceed 3.6 V.
The lithium iron phosphate battery according to claim 1, wherein the negative electrode membrane has a compact density of from 1.4 g/cm 3 to 1.8 g/cm 3 .
The lithium iron phosphate battery according to claim 1, wherein the cyclic carbonate containing a double bond is one or more selected from the group consisting of vinylene carbonate and ethylene carbonate.
The lithium iron phosphate battery according to claim 1, wherein the cyclic disulfonate is selected from the group consisting of methylene methane disulfonate, ethylene ethane disulfonate, and propylene glycol methane disulfonate. One of Or several.
A lithium iron phosphate battery according to claim 1, wherein in said electrolyte:

The mass percentage of the cyclic carbonate containing a double bond is 0.5% to 4%, preferably, the mass percentage of the cyclic carbonate containing a double bond is 0.5% to 3%;

The cyclic disulfonate has a mass percentage of 0.2% to 2%, preferably, the cyclic disulfonate has a mass percentage of 0.2% to 1%.
The lithium iron phosphate battery according to claim 1, wherein the electrolyte has a conductivity of 25 ° C of 8 mS/cm to 11 mS/cm.
The lithium iron phosphate battery according to claim 1, wherein the electrolyte has a viscosity at 25 ° C of 2 mPa·s to 4 mPa·s.
The lithium iron phosphate battery according to claim 1, wherein the organic solvent comprises a mixed solvent of a cyclic carbonate and a chain carbonate, and the organic solvent contains a chain carbonate having a methyl group. And the mass percentage of the chain-like carbonate having a methyl group is 40% or more based on the total mass of the organic solvent of the electrolytic solution.
The lithium iron phosphate battery according to claim 9, wherein the organic solvent further comprises a carboxylic acid ester, and the content of the carboxylic acid ester is less than or equal to 30% of the total mass of the organic solvent of the electrolytic solution.